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Lesson Note

Subject: Agricultural Science
Topic: Husbandry Of Selected Crops.

Subtopic: Production of Orange (Citrus sinensis)
Lesson Objectives: At the end of the lesson, learners should be able to:

  1. Explain the process of producing citrus,
  2. State the different varieties of citrus,
  3. Describe the different parts of a citrus body.

Lesson Discussions

Scientific Classification

Origin: The origin is believed to be from the drier tropics of South-East Asia. It is cultivated now in both tropics and sub-tropics.

Uses: It is grown for the production of juicy fruits and drinks. Traditionally it has medicinal herbs.

Varieties: Citrus spp belong to the members of rutaceae family and include:
Common name Botanical name
i. Sweet orange Citrus sinensis
ii. Tangarine/Mandarin orange Citrus reticulata/nobilis
iii. Grape fruit Citrus paradisi
iv. Pummelo/shaddock Citrus grandis
v. Lemon Citrus limon
vi. Citron Citrus medica
vii. Sour orange Citrus aurantium
viii. Lime Citrus aurantifolia

General Morphology:
Heights of Citrus spp vary according to the species. Some species develop into shrubs while others grow into tall trees of over 10m high.
It has tap root system with the root growth alternating with that of the stem. Some species gave thorns that are adjacent to the leaf axils. Shoot development is not continuous but in three well defined cycles in a year.
The leaves are unifoliate with wings being present in some species. The flowers are borne singly or in clusters in leaf axils of new shoots. Usually Citron and lemon have both male and female flowers. In all species, there is a nectary on which the ovary is mounted. The ovary has 8 – 15 united carpels and each carpel has two rows of ovules. Most species are both self and cross pollinated.
The fruit of citrus is called a “hesperidum” and the form varies.
The rind has the following parts:
i. Exocarp – coloured outer portion.
ii. Mesocarp – white inner part
iii. Endocarp (rag) – lies between the white inner part and the carpel.

Transverse section of an orange fruit

The rag has a hairless structure developing from the inner side and at time of fruit maturity, become filled with juices – citric acid and sugars. Some cultivars e.g navel oranges are seedless. Seed producing cultivars vary in the number of seeds and most are polyembryonic, that is, seeds develop from both sexual and asexual embryo.

Ecology: Citrus thrives best in a moderately acidic soil that is well drained. It is a sun loving crop and requires not less than 1000mm of annual rainfall that is evenly distributed.

Husbandry: Citrus is propagated by seedling and by vegetative means. While well adapted cultivars grow from seeds to maturity those that are susceptible to prevalent diseases such as tristeza and gummosis are usually budded (and in rare cases grafted) to the root stocks of the resistant cultivars. The selection for the desirable root stocks is based on compartibility of cultivars and resistance to especially gummosis and then tristeza. Rough lemon and sour orange are commonly used stocks. Sweet orange is rarely used. Sour orange is a root stock compartibility with other species except lime. Rough lemon is compartible with tangarine, grapefruit and limes. Sweet orange is resistant to tristeza and is used as a root stock for lemon. Sour orange and rough lemon are resistant to gummosis.

Nursery Preparation: Healthy seeds from desirable species are selected, treated and sown at a depth of about 2.5cm at a spacing of 15 by 15cm. A standard seed bed measures 7.5 × 1.2 m with a gap of 0.6m. The pre-nursery seed bed has to be loamy. It has to be watered in the early hours of the day.
The seedlings have to be transplanted from the pre-nursery seed beds to the polybags that had been filled previously with rich top soils. The spacing of the polybags should be 30cm by 30cm. It is advised that the end of the polybags nursery stage should be timed to coincide with the early rains to help reduce the need for watering and shock on the transplanted seedlings.

Budding of Citrus Seedlings: When seedlings in the polybags attain the thickness of a pencil, they should be budded. The bud wood (Scion) is obtained from the young branches of disease free desirable citrus trees. Resistant and compartible species serve as the root stock. An inverted T-shaped incision is made on the root stock at about 30cm from the level of soil surface. This height is chosen to reduce infection. The Scion is slipped into the T-shaped incision in such a way that their cambial surfaces unite. It is therefore, tied with raffia or budding tape. This ensures that the tissues of the Scion and the stock unite and keeps off pathogens. The area is also covered with wax to exclude pathogens.
The bud remains green if it has “taken” and at this point “half capping” is done
This refers to cutting half of the root stock stem above the budded area and then bending it slightly. After about 21 days, if the budding had been successful, the bud progressively grows out. At this point, “full capping” is carried out, that is, cutting off completely the part of the root stock above the budded area. Cover the cut surface with tar or point to avoid infection.
Ensure that no new flush grows out below the budded area by rubbing off new outgrowths. Two or more species of citrus could be budded to the same root stock.

Transplanting of Citrus: Prior to transplanting the permanent site has to be cleared and the planting spaces marked with pegs. Planting holes are dug to accommodate the ball-of-earth bearing the citrus too. Carefully, tear off the polybag and gently place the ball of earth into the planting hole. Ensure that it is erect and then cover the surrounding hole with the preserved top soil without destroying the citrus roots. Apply rustica (NPK 12:12:17) 10 – 15cm away from the plant and mulch. Remove weeds manually or by herbicidal application
Prune until such a time when the branches of the citrus species are fully developed.

Maturity and Harvesting: maturity of citrus fruits takes 7 – 14 months from pollination. The citrus plants start producing fruits about 4 years from transplanting. September to November is the peak period of production. Orange fruits are green in colour and on ripening, change to light yellow (orange) colour. Mature green fruits have to be harvested if fruits have to be transported to distant markets. Fruits are harvested with hand, short and long handled sickles or even with poles with hooked end.
Pests: Aphids, mealybugs, and grasshoppers are common insect pests.

Diseases: The most prevalent disease are gummosis, scab and tristeza.

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  1. Explain the process of producing citrus.
  2. State the different varieties of citrus.
  3. Describe the different parts of a citrus body.

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